FROM THE TERTIARY LAVAS AROUND BEN MORE, MULL. 15 



difficult to determine under the microscope, as in many cases they possess low 

 refractive indices, weak double refraction, and anomalous optical characters which 

 often make it impossible accurately to determine individual members by examination 

 of thin sections alone. In the present case, however, the number of zeolites is 

 limited and the individual species can be readily identified. 



Scolecite occurs in long fibrous crystals (refractive index, l - 5) which are weakly 

 birefringent, extinguish obliquely, and have the zone of elongation negative. Trans- 

 verse sections across the fibres have very low birefringence, and in many cases show 

 the emergence of an acute, negative bisectrix with a low axial angle. These char- 

 acters distinguish it at once from thomsonite (and natrolite), which shows much 

 stronger birefringence, straight extinction, and the emergence of an acute, positive 

 bisectrix perpendicular to the direction of elongation. The axial angle is large ; and 

 the optic axial plane is perpendicular to the zone of elongation, which is consequently 

 sometimes positive and sometimes negative : in the former case the section shows 

 low birefringence and is perpendicular to the acute bisectrix. The only other zeolite 

 observed in the slides is heulandite, and it is of rare occurrence. It shows good 

 cleavage and can be always determined by the fact that cleavage flakes show the 

 emergence of a positive, acute bisectrix. 



The prehnite is characterised by good cleavage, high refraction and birefringence, 

 straight extinction and positive optical character. It occurs in large, compact, 

 colourless plates and spherules which usually show wavy extinction under crossed 

 nicols. They are often built up of sectors which radiate from a point in the plate, 

 and, on being rotated under crossed nicols, the section shows two dark hyperbolae 

 which close up and open out in directions at right angles to each other. Idiomorphic 

 crystals with uniform extinction have not been found, but areas large enough to 

 show the characteristic optical characters are quite common. 



The determination of the epidote and the albite offers no special difficulty, 

 as they are quite typical. The albite, however, is sometimes so turbid that it is 

 difficult to identify it, and in such cases the mineral was always separated from 

 the hand-specimen, powdered, and examined in oil of refractive index 1*534 under 

 the microscope. No attempt was made to identify the different kinds of chlorite 

 present. The mineral varies in colour from pale yellowish-green to deep green, 

 the latter variety being distinctly pleochroic. It occurs usually in small, vermicular 

 growths, but is also found in fibrous and platy forms. 



With the help of the sections it* is possible roughly to divide the vesicles into 

 two main types : — 



(a) Those vesicles whose junctions with the rock are sharp, and which 

 do not contain any of the igneous minerals of the rock. 



(b) Those vesicles whose junctions are not so sharp, and which contain, 

 especially in their outermost zones, igneous minerals similar to those found 

 in the rock. 



